Wednesday, March 13, 2013

Temporal aliasing and bendy water - nice!

I remember as a kid watching a western movie and being amazed to see the stage coach wheels appear to rotate backwards as the coach moved forwards.  The wheels weren't actually moving backward.  It was an illusion caused by a combination of the speed of the wheel spinning, the wheel's symmetry  the movie making process and my brain.  As you know, movies are a collection of still pictures shown in rapid succession.  Your brain puts these images together and essentially fills in what must be going on between the frames.  For movies, these frames are coming at you 24 times per second.  In the case of the wagon wheels imagine that during that 1/24th of a second, the wagon wheel moves forward just slightly.  In that case, your brain fills to assume that the wheel moved forward a little bit.  Now imagine that the wagon moves at a speed such that, between each frame of the movie, the wheel rotates one eighth of a rotation.  And, lets suppose that the wheel has eight spokes in it.  Then, from one frame to the next, the spokes of the wheel would line up.  That would mean that the wheel would appear to not rotate at all from one frame to the next.  If it moved just less than 1/8 of a rotation, from one frame to the next, your brain will assume that the wheel has moved backwards slightly.  Putting these together caused the effect that I noticed as a kid; a forward moving coach with backward moving wheels.  This effect is called temporal aliasing.  The video above shows this effect with varying camera shutter speeds.  The faster shutters speeds more effectively freeze the movement and give a stronger effect.

The amazing video below uses this temporal aliasing to make water appear to do crazy things.  By connecting a hose to a speaker, it can be wiggled at a speed that matches up with the speed of the frames of the video.  This effectively freezes the water in mid vibration at the same point each frame.  If you were on location, you would see the water spraying out in fan shape away from the end of the hose, not what we see on the video.  Since the video is in phase with the vibrations, the water appears frozen in mid stream (literally), and the effect is really cool.  Of course as they speed up and slow down the vibrations, the water appears to move up or down accordingly.  What fun!